Conventionally, a Schottky diode includes a heavily-doped semiconductor substrate, typically made of single-crystal silicon. A second layer covers the substrate. The second layer, called the drift region, is less heavily-doped with impurities having carriers of the same conducting type as the substrate. A metal layer or a metal silicide layer forms a Schottky contact with the lightly-doped drift region and forms the diode anode.
Two opposing constraints arise when forming a unipolar component such as a Schottky diode. In particular, the components should exhibit the lowest possible on-state resistance (Ron) while having a high breakdown voltage. Minimizing the on-state resistance imposes minimizing the thickness of the less doped layer and maximizing the doping of this layer. Conversely, to obtain a high reverse breakdown voltage, the doping of the less doped layer must be minimized and its thickness must be maximized, while avoiding the creation of areas in which the equipotential surfaces are strongly bent.
Various solutions have been provided to reconcile these opposite constraints, which has led to the development of trench MOS-capacitance Schottky diode structures, which are referred to as Trench MOS Barrier Schottky (TMBS) diodes. In an example of such devices, trench regions are formed in the upper portion of a thick drift layer that is less heavily doped with impurities of the same conductivity type than the underlying substrate. The trench regions are filled with a MOS structure. An anode metal layer is evaporated to cover the entire surface and forms a Schottky contact with the underlying drift region.
When reverse biased, the insulated conductive areas cause a lateral depletion of charge into the drift region, which modifies the distribution of the equipotential surfaces in this layer. This enables increasing the drift region doping, and thus reducing the on-state resistance with no adverse effect on the reverse breakdown voltage.
A key issue for achieving a high voltage Schottky rectifier is the design of its termination region. As with any voltage design, the termination region is prone to higher electric fields due to the absence of self multi-cell protection and the curvature effect. As a result, the breakdown voltage is typically dramatically reduced from its ideal value. To avoid this reduction, the termination region should be designed to reduce the crowding of the electric field at the edge of the device (near the active region). Conventional approaches to reduce electric field crowding include termination structures with local oxidation of silicon (LOCOS) regions, field plates, guard rings, trenches and various combinations thereof. One example of a Schottky diode that includes such a termination region is shown in U.S. Pat. No. 6,396,090.
FIG. 1 shows a simplified, cross-sectional view of the active and termination regions of a TMBS Schottky diode of the type shown in U.S. patent application Ser. No. 12/724,771. The active region includes a semiconductor substrate 100B that is heavily doped with a dopant of a first conductivity type (e.g., n+ type). A first layer 100A is formed on the substrate 100B and is more lightly doped with a dopant of the first conductivity type (e.g., n− type). Trenches 110 (only one of which is shown) are formed in the first layer 100A. The trenches 110 are lined with an insulating layer 125 and filled with a conductive material 140 such as doped polysilicon. A metal layer 165 is formed over the exposed surfaces of the conductive material 140 and the first layer 100A, thereby forming a Schottky contact 160 at the interface between the metal layer 165 and the first layer 100A. A cathode electrode (not shown) is located on the backside of the semiconductor substrate 100B.
The termination region of the TMBS diode shown in FIG. 1 includes a termination trench 120 that extends from the boundary 112 with the active region toward an edge of the semiconductor substrate 100B. A MOS gate 122 is formed on a sidewall of the termination region adjacent to the boundary 112 with the active region. The MOS gate includes an insulating material 128 and a conductive spacer 122. The insulating material 128 lines the sidewall against which the conductive spacer 122 is located and the portion of the first layer 100A adjacent to the sidewall. The conductive spacer 122 covers the insulating material 128. A termination oxide layer 150 is formed in the termination trench 120 and extends from the conductive spacer 122 toward the edge of the device. The metal layer 165 located in the active region extends into the termination region and covers the conductive spacer 122 and a portion of the termination oxide layer 150 to thereby define a field plate.
Unfortunately, for high voltage applications these conventional designs for the termination region have had only limited success because the electric field distribution at the surface of the termination region is still far from ideal. Because of the limited length of the drift region, the electric field rises rapidly at the end of active region due to the asymmetry. As a result the breakdown of the device is dominated by edge breakdown.
The conventional device shown in FIG. 1 has been driven to 200V, but at this point its performance is already degrading because of the premature breakdown at the surface of the termination region. Consequently the reliability of this design largely depends on the end position of the field plate 165 in the termination regions. Normally, the metal wet etching process used in the formation of the field plate 165 can only be controlled to a precision within about ±6 μm, and this variability can have a significant impact on the device's reverse blocking voltage. For instance, a short field plate will exaggerate the electric field near the corner of the last active cell, resulting in premature breakdown. On the other hand, a longer field plate that extends to a point near the remote spacer can degrade the breakdown voltage as well, while also causing mechanical stress at its elongated metal end.
TABLE 1Breakdown voltage vs. metal field plate length of conventional TMBSterminationExtended Metal Length Variation (μm)−4−20+2+4+6+8Breakdown235277278276271269261Voltage, Vbr (V)Breakdown−15.5−0.72—−0.72−2.52−3.24−6.14Fluctuation (%)
Table 1 shows the variation in breakdown voltage as a function of the length of the metal field plate. The data were obtained from a simulation of a drift layer designed for high breakdown voltage TMBS devices with a 20 μm termination trench. It should be noted that the breakdown voltage of the unit cell with the same parameters of the drift region is 375V, and, as the Table shows, the highest breakdown voltage achievable with the conventional termination design is 74% of the ideal value.